In a multiple tube and shell heat exchanger, tube sheets are used to separate the channel sections from the shell section. Tubes are inserted through holes in the tube sheets which allow a normally hot fluid to be passed from the inlet channel section through the tubes to the outlet channel section, while segregating this fluid from the heat absorbing fluid passed through the shell space. Typically, the tubes are sealed to the tube sheets by rolling and brazing or welding. Such heat exchangers are in common use.
Since the tube sheets are exposed to both the hot and heat absorbing fluids, they must be constructed of material or materials which are compatible with both fluids. In cases where the hot fluid is corrosive, the tube sheets are often clad with resistant material such as titanium; the tubes are constructed of the same material. The base metal, usually carbon steel, supplies the required strength to the tube sheets while titanium, which has a low allowable stress at elevated temperatures, provides the necessary corrosion resistance. To prevent deterioration of the tube sheet, the corrosive fluid must not be allowed to contact the base metal. Therefore, the tubes are welded directly to the cladding material on the channel side of the tube sheet.
Because of the required tight spacing of the tubes on the shell side of the tube sheets, it is not possible to weld the tubes to a shell-side cladding. Therefore, in applications where it is necessary to provide corrosion resistance to both sides of the tube sheet, it has been necessary to construct the tube sheet from a solid piece of titanium. At elevated temperatures (120.degree.-300.degree. C.), titanium has a very low allowable stress, requiring an extremely thick, heavy and expensive tube sheet to withstand high pressures (300-3,000 psig). A tube sheet of sufficient thickness to withstand this pressure is usually impractical, since the cost of the metal is high and drilling precision holes through thick metal presents difficulties.
In a heat exchanger e.g. that requires both titanium tubes and shell, titanium tube sheets are required. At high temperatures, titanium becomes weak and if a large pressure differential exists across a tube sheet, it may have to be designed to be very thick. This thickness for a tube sheet 18" in diameter operating at 290.degree. with a design pressure differential of 2000 psig would be more than ten inches thick.
This would be a very expensive piece of metal and further, it becomes very difficult to drill straight holes through a piece of metal ten inches thick.
In addition, welding the tubes to the shell side of the tube sheet (whether cladding, lining or base metal) can be virtually impossible due to close tube spacing, and further is undesirable because subsequent removal of a tube or tubes for repair or replacement is difficult.
An object of this invention is the construction of an assembly e.g. heat exchanger tube sheets which are corrosion resistant on both shell and channel sides, preventing contact of corrosive fluid with any non-resistant materials.
A specific object is to reduce the amount of expensive corrosion-resistant metal required by utilizing thin layers of cladding or lining over both sides of the tube sheet base metal, using resistant sleeves, and resistant tubes in the sleeves.
An additional object is to make removal of any one or more tubes possible without the removal of weld metal from the shell side of the tube sheets.